RADICAL-CATION CYCLOADDITION REACTIONS OF 2-VINYLBENZOFURANS AND 2-VINYLFURANS BY PHOTOINDUCED ELECTRON-TRANSFER

2-Vinylbenzofuran (2a) and 2-isopropenylbenzofuran (2b) can be success fully employed in cycloaddition reactions involving photoinduced elect ron transfer with 1,3-cyclohexadienes 4, styrenes 5 and acyclic 1,3-di enes 6, yielding [4 + 21]-and/or [2 + 2]-cycloadducts. 2,4,6-Tri(4-mel hoxyphenyl)pyrylium telrafluoroborate (la) acts as an efficient sensit izer. Products are benzofuryl substituted cyclobutanes, 1,2,3,4-tetrah ydrodibenzofurans, 2-benzofuryl bicyclo[2.2.2]octenes, and benzofuryl substituted cyclohexenes. Of 2-vinylfurans, only the 5-bromo substitut ed derivative 3 reacted with 1,3-cyclohexadiene. The 2-vinylbenzofuran s behave like styrenes in these photoinduced cycloaddition reactions m ostly generating intermediate cyclobutyl radical cations. 1,3-sigmatro pic C- and H-shifts taking place at the radical cation stage are likel y in these reactions. The mechanisms are discussed in detail, supporte d by quantum chemical calculations.